Polylactate (PLA) is a typical biodegradable polymer derived from lactate that is highly applicable commercially and biomedically. Although preparation of PLA presently involves polymerization of lactate produced by fermenting microorganisms, only PLA with a low molecular weight of about 1000 to 5000 daltons is obtained by direct polymerization of lactate. In order to synthesize PLA with a molecular weight of 100,000 daltons or higher, PLA with a low molecular weight obtained by direct polymerization of lactate may be polymerized using a chain coupling agent. In this method, however, the entire process becomes complicated due to addition of an organic solvent or a chain coupling agent, which is not easy to remove. A presently commercially available process of preparing high-molecular weight PLA may include converting lactate into lactide and synthesizing PLA using ring-opening polycondensation of lactide rings.
When PLA is synthesized by chemical synthesis of lactate, a PLA homopolymer is easily obtained, but a PLA copolymer composed of various types of monomers is difficult to synthesize and commercially unavailable.
Meanwhile, polyhydroxyalkanoate (PHA) is polyester stored by microorganisms as energy or a carbon source when there are excessive carbon sources and a lack of other nutrients, such as phosphorus (P), nitrogen (N), magnesium (Mg) and oxygen (O), etc. Since PHA has similar physical properties to a conventional synthetic polymer from petroleum and exhibits complete biodegradability, it is being recognized as a substitute for conventional synthetic plastics.
In order to produce PHA using microorganisms, an enzyme for converting microbial metabolic products into a PHA monomer, and PHA synthase for synthesizing a PHA polymer using the PHA monomer, are needed. When synthesizing PLA and PLA copolymer using microorganisms, the same system is required, and an enzyme for providing lactyl-CoA is needed in addition to an enzyme for providing hydroxyacyl-CoA, which is an original substrate of PHA synthase.
Therefore, in order to provide lactyl-CoA, the present inventors used propionyl-CoA transferase from Clostridium propionicum and a mutant of PHA synthase from Pseudomonas sp. 6-19 using the propionyl-CoA transferase as a substrate, thus could successfully synthesize PLA and PLA copolymer as disclosed in Korean Patent Application NO. 10-2006-0116234.
However, it was reported that when propionyl-CoA transferase from Clostridium propionicum, which is an enzyme for supplying lactyl-CoA, is highly expressed in E. coli by a very potent promoter, serious metabolic disorder occurs, thus inhibiting cell growth (Selmer et al. reported in Eur. J. Biochem. 269:372, 2002). Also, since the codon usage of a gene for encoding propionyl-CoA transferase from Clostridium propionicum is quite different from that of E. coli, it may be very difficult to normally express propionyl-CoA transferase. Accordingly, in order to synthesize PLA and PLA copolymer more efficiently than conventional systems, it is very important to introduce propionyl-CoA transferase, which smoothly provides lactyl-CoA and is expressed enough not to greatly inhibit cell growth.